The invention relates to a circuit arrangement for igniting and operating a discharge lamp by means of a high operating frequency drive voltage, said circuit comprising a half-bridge commutator having two switching elements each having an emitter electrode and a control electrode, said switching elements being alternately switched to a conducting state by means of a resonant control circuit which is coupled to an externally driven oscillator via a transformer, said resonant control circuit comprising capacitor means forming part of said oscillator.
Such a circuit is known from international patent application WO-A-00/21341. The known circuit is used to ignite and operate an electrodeless gas discharge lamp. In the known circuit the half bridge commutator and the resonant control circuit form together a power stage supplying power to the load. In this prior art circuit arrangement, the amplitude of the drive voltage applied to the power stage supplying power to the load, in the prior art the electrodeless gas discharge lamp, is frequency-dependent due to the fact that the power stage comprises a resonant transformer. From a technical point of view, the transformer can not be omitted from the circuit arrangement because of the high operating frequency, for electrodeless gas discharge lamps generally 2.5-2.8 MHz. The amplitude variations give rise to loss of zero voltage switching in the power stage and high driver losses when the amplitude is too high and to increased conduction losses in the power stage when the amplitude is too low. This forms a technical problem.
It is an object of the invention to design a circuit arrangement of the type specified in the above-mentioned preamble, in such a way that this technical problem is solved. Furthermore, it is an object of the invention to make sure that the operating frequency of the converter is kept exactly equal to the resonant frequency of the load circuit. It should be appreciated in this respect that the resonant frequency may vary appreciably due to component tolerances and temperature effects. In the circuit arrangement, the voltage-controlled oscillator forms part of a frequency loop in which the phase difference between the half-bridge voltage and the circuit voltage is kept at a value of exactly 900. Thus, the VCO must be kept in line therewith. This is the reason why the gate driver, in a specified frequency range, should supply a voltage having a sufficiently constant amplitude.
Furthermore, it may be desirable, especially in the igniting phase of the discharge lamp, to use a frequency differing from the frequency used for normal use. Also under such circumstances the gate driver must operate properly.